The present invention pertains to globe valves especially globe valves for use in cryogenic service.
Cryogenic fluids e.g. liquid hydrogen, liquid oxygen, liquid nitrogen, liquid helium, and liquid argon are delivered in large over the road tanker trucks to a customer location where the cryogenic liquid is off loaded from the truck into a large storage tank, as is well known in the art.
Both the tank truck and the storage receptacle or customer station, as they are known in the industry, include flow control valves which are used to both fill and dispense the cryogenic liquid.
Conventional valves are of the globe type having an extended stem to space the bonnet assembly of the valve from the valve seat, so that cryogenic liquid leaking along the shaft is warmed prior to coming in contact with the valve actuator so that the valve actuator does not freeze and render the valve inoperative.
Extended stem globe valves for use in cryogenic service can be either manually or pneumatically actuated to control flow into or out of the trailer or tank. The valves can also be vacuum jacketed to enhance insulation of the valve and thus further minimize the effect of cryogenic fluid leakage inside the valve.
Conventional extended shaft globe valves incorporate a plug body/fluorocarbon disk sandwich held together by a bolt/stud assembly. This type of assembly requires a nut and washer on the bottom of the plug assembly in order to hold the fluorocarbon plastic material (KEL-F) disk to the plug body. It is conventional for the nut to be staked in place, or a cotter pin is inserted through the bolt stud to prevent the assembly from coming apart during the service life of the valve. The problem with this type of assembly is that the nut and washer assembly is located below the fluorocarbon plastic material and is therefore exposed to the seat flow/pressure boundary of the valve. If the nut comes loose or falls off during the service life of the valve several problems can occur. One problem is that the nut/washer/fluorocarbon plastic material seat can all be lost and total control of the seating of the valve is lost. In addition, if this happens the compressed stream with which the valve is used will become contaminated and a potential source of a spark has been introduced into the system. Potential spark or ignition sources are a definite hazard where the valve is used with oxygen. If a cotter pin is used it is probable that only the nut will come loose but the assembly will stay attached. This type of design requires that the assembly have a through drilled orifice for attachment and, if the assembly becomes loose, there is potential for multiple leak paths through the assembly itself, even if the valve is in the "full-off position" where a greater amount of leakage can occur.
In addition, in conventional extended stem or shaft valve designs the ends of the shaft are generally flat and can be attached with a dovetail arrangement to both the plug body seat assembly on one end and the actuator on the other end, to permit disassembly of the valve. It is also possible to use rigid threaded connections on either end of the extended stem or shaft. The flat end design of conventional valves permits high heat leakage by conductance across the shaft. With flat ends rigidly connected to the valve plug assembly there is a potential for eccentric axial loading from the top of the stem to the plug assembly, which in turn can cause premature wear of the internal components of the valve.
It is also a common problem with current valve designs that when a valve is installed near a dynamic pressure application, i.e. such as the outlet side of a cryogenic liquid pump, that the valve "top works" in the area of the bonnet assembly and will begin to accumulate frost. The frost accumulation is a sign of minor failure or leakage in the "top works" of the valve. It is believed that a large volume of gas in the free space is the contributing factor to this type of premature failure. As the pressure surges inside of the piping and the associated valve, these surges are being introduced into the large gas pocket inside the valve and a combined pressure swing along with new colder gas being introduced by the percolating effect introduce excessive cold temperatures and wear on the valve packing.
Furthermore, it is conventional for extended stem cryogenic valves to incorporate a chevron style polytetrafluoroethylene(TEFLON) or Grafoil Packing. The packing acts as a seal/barrier between the external atmosphere, which is at relatively low pressure and warm and the higher pressure, cold temperature cryogenic process service. This is commonly called a "packed stem". Usually, this packed stem has a nut or other means to permit a user to tighten the packing during the normal service life of the valve. The packing acts as a seal between the process stream and the atmosphere, as well as for providing for relative movement of the valve hand wheel or actuator stem and thus is commonly referred to as a dynamic seal.
Several problems exist with the chevron style packing used with extended stem cryogenic valves. First, the polytetrafluoroethylene and graphite materials have less than desirable friction coefficients as they wear due to the dynamic stem rotation and linear actuation of the stem. Furthermore, in cold weather, the polytetrafluoroethylene (TEFLON) shrinks to a much greater degree than the surrounding materials. This relative shrinkage can produce leakage and frosting at this location. Typically this is resolved in the field by tightening down on the packing nut. Although tightening of the packing nut will temporarily resolve the leak the packing has now been over tightened. Over tightening of the packing causes the Teflon to cold flow and when the valve warms up again the packing will now tend to bind the valve stem and the binding can create premature wear thus greatly reducing the service life of the packing assembly. In conventional valves the hand wheel is fixed to the upper stem piece by an internal or external thread thus, rotation of the handwheel introduces axial movement for linear valve plug movement.